A conventional gas boiler includes a ventilator 10 that introduces external air and supplies air necessary for combustion, a gas valve 20 that adjusts supply of gas for combustion that is mixed with the air introduced by the ventilator 10, a burner 30 that combusts a mixture of the gas for combustion and the air supplied by the ventilator 10, a combustion chamber 40 in which the combustion of the burner 30 occurs, a heat exchanger 50 in which a heat exchange between combustion gas generated by the burner 30 and hot water occurs, and an exhaust gas hood 60 through which the combustion gas passing through the heat exchanger 50 is discharged.
In the conventional gas boiler constructed as described above, when an output of a boiler is changed, it means that an output of the burner 30 is changed. In order to change the output of the burner 30, the amount of air and the amount of gas have to be changed in proportion to the output.
However, the conventional gas boiler has problems in that since the ventilator 10 that introduces external air and the gas valve 20 that adjusts the amount of gas for combustion which is supplied are separately disposed and an additional unit for controlling the amount of gas supplied by the gas valve 20 in proportion to the amount of air supplied by the ventilator 10 has to be provided, a structure of a device is complex and it is difficult to accurately control a mixture ratio of the air and the gas for combustion according to a preset output of the boiler.
A method of changing the amount of air which is a method of changing the number of rotations of the ventilator 10 involves rotating the ventilator 10 at a high speed when a high output of the boiler is necessary and rotating the ventilator 10 at a low speed when a low output of the boiler is necessary.
In this case, as the ventilator 10 is changed from a high speed rotation to a low speed rotation and the number of rotations is reduced, a wind pressure of air supplied by the ventilator 10 is also reduced.
In this case, when the wind pressure is reduced to a value less than or equal to a predetermined value, although the ventilator 10 rotates at a low speed, the ventilator 10 does not supply air.
Accordingly, there is a limitation in reducing an output of the boiler using only a method of reducing the number of rotations of the ventilator 10, reducing the amount of air which is supplied, and proportionately reducing the amount of gas for combustion supplied by the gas valve 20 in the conventional gas boiler. Accordingly, a range of a turn-down ratio (TDR) of the boiler may not be increased.
The TDR refers to a ‘ratio of a maximum amount of gas consumption to a minimum amount of gas consumption’ in a gas combustion device in which the amount of gas is variably adjusted. The TDR is limited by the minimum amount of gas consumption for maintaining a stable flame. In a gas boiler, as the TDR increases, user convenience during heating and hot water supply may increase, a deviation in temperature control may decrease, and the durability of a device may be improved.